So obviously I made this very quickly and it may contain mistakes above, but do not let this take away from the overall build.

Basicly when I started searching for builds on Riven, everything was offensive, which is not at all how I prefer to play my melee dps. I experiemented and have played loads of games, and have narrowed it down to a defensively based build that allows you to put out needed dps,while being able to jump into teamfights as one of the tankiest characters in the game, offering a large amount of cc and dmg.
The runes are pretty standard on riven, except I take ad marks instead of the common armor pen as it scales with all your abilities, giving you great early game damage. The others just add to your tankiness. This Masteries give you that extra durability that will allow you to play the role of this build, which is jumping in and completely disrupting the other team.

With this build, you need an awful lot of farm. I'm warning you, do not play Riven when you will not receive a solo lane, well atleast not with this build. Assuming you have a solo top, Most commonly I will start with a Growth Pendant and a Health pot, giving you incredible sustainability early game, with the ability to free farm and scale into a strong mid/late game.
Start with your q, for mobility and some harass. If you take the dmg marks and quints, it is often possible land a full q, auto, q, auto, q, auto on the enemy champ as generally they underestimate your dmg output and attempt to counterharass. Believe it or not, this will usually bring them to below or just at half health, giving you a massive advantage from their mistake. If you are not able to execute this, no worries, your still a strong top.
Next take your w for last hitting those minions that you would otherwise not beable to get, and even stronger harass. With your regrowth pendant, it is viable to be very aggresive and wear down your enemy, either zoning them off of cs or sending them back.
Then you should take your e, giving you a dash and shield, and making your harass quicker, and disabling your enemies counter harass. Dash in, q, w, then continue your q and autoattacks if they retreat. Note: Finishing all 3 of your q's is not neccessary, as your shield will wear off and you are vulnerable to counter harass that will generally out dps your q combo. So recap, its just important to dash in, get a q and w off, along with an autoattack, then back off leaving you with some decent dmg and 0 return dmg due to your shield. You will slowly wear them down and gain an advantage, and put them at a defensive allowing you to free farm. Continue lvling your w and e similtaneously depending on who you are going against, and if you are very comfortable you can grab another lvl in q to be more aggressive, but usually i dont/
By the time you go back for the first time, you should have enough for boots, and atleast a vampiric sceptar. If you clearly dominate your lane and got good farm and maybe a first blood, if you have enough for phage, definatly grab that instead. DO NOT ROAM ON RIVEN WITH THIS BUILD. The goal is to get as much farm as possible, and get your warmogs/frozen mallet asap. Usually I'll get the phage as above ^, then work towards warmogs. Don't start saying rushing warmogs is useless, try it on Riven then tell me. By the time you get these two items you should be working on atmas, but laning phase will be near the end and dragon fights etc will start to ooccur.

By this point you will probibly have your tier 2 boots, Frozen Mallet, Warmogs, and prob a vampiric scepter. The type of boots completely depends on the enemy team, you decide. Mercs would be basic but if theyre ad heavy the dodge ones, if they got alot of mobility maybe the +3 movement. Anyways, fights will start breaking out between lanes and at dragon as people start roaming. Trust me, you are very strong at this point, so make yourself useful and be in these fights. You have great dps, and are vertually unkillable with your rank 5 e and stacked up warmogs, along with frozen mallet. In these small 2v2, 3v3 skrimages, you will completely dominate them. Unless you were denied in lane, you offer way too much cc, dmg, and durability for anyone to do anything against you, and you will most likely win these fights, giving your team a gold advantage, with possibly dragon or a tower push.

So by the time you get atmas, with the build above you will probibly have around 4k health, granting you about 80 dmg, which is alot... You will be probibly one of if the the tankiest in the game, and have great initiations. During initiation, you will probibly know what to do after playing her several times, as the best combos vary with their positioning. There's so many different scenarios that you just have to know, but the goal of teamfights with Riven is getting as many enemies as possible with your cc, basicly valueing squishy dps over tanks. If they are positioned properly with their squishies in the back, you are gonna have to decide whether your squishies are in jepordy and you should use your w to allow them to get into better position, or to run directly through their team to get their squishies with your w and leave them vulnerable to you and the rest of your team. The use of your q is alot different than in small skirmishes, as if they are clumped up, go ahead and spam it getting the push back off and dealing the aoe dmg, then sorta autoattack the rest down (your passive will stilll have 3 hits on it)
As your ultimate goes, always activate it when a teamfight is eminate, as the stronger part of it is the passive giving your abilities longer range and dmg. Activate it some time into the fight where most are down to half health or so, as you will get the most dmg out of it rather than just last hitting 1 person at the end. By the way get a bloodthirser after your atmas, forgot to put that in and don't want to deal with ti.

Yes I realize building Riven tanky is not what most people think is best, but just try it and I'm sure you'll be impressed. It's probibly closest to a tanky udyr role with quick short range gap closers and cc. In my opinion, this is by far the best way to build her. If your going for melee dps... just use xin or yi, they can actually backdoor which is a large purpose of that role.
Remember when playing her, especially in this way, be sure you have a solo top before the game starts, or you will not have enough farm for this build. Once you get a few solos under your belt with her, it should be quite easy as she's very strong once you get the sequence down. Anyways, I hope this helps and you should try it out before trolling on it. Sorry about all the errors in the upper part^, one of my first builds and done a very short time.